The present invention relates generally to microwave filters, and more particularly, to improved miniaturized single mode and dual mode filters for use at relatively low microwave frequencies, such as for use in wireless and satellite communication systems.
Currently available wireless and personal communication system (PCS) devices operate at relatively low microwave frequencies, typically in the 800-2600 MHz frequency range. Previously available microwave filters for use in wireless and PCS devices have employed waveguide cavities, coaxial resonators and dielectric resonators. However, such conventional microwave filters are relatively large, which means that the devices containing them are unnecessarily large.
Helical resonator filters are well-known and widely used at lower microwave frequencies. As a microwave structure, the helix (helical resonator) is used for many applications in traveling-wave tubes, antennas, and delay lines. At lower frequencies, helical resonators operating in a fundamental mode (resembling a coaxial, quarter-wave resonator) are often used. Unfortunately, the Helmholtz equation is not separable in helical coordinates, as is discussed in a book by R. E. Collin, entitled "Field Theory of Guided Waves", McGraw-Hill Book Company, New York, 1960. Furthermore, to the inventors' knowledge, an analytical, rigorous solution for the general helical resonator has not yet been obtained. Various simplifying assumptions (e.g. sheath helix) have been used successfully in the past, but significant theoretical work needs to be completed to explore the full potential of the helical resonator.
Dual mode structures are widely used to realize high performance filters 20. Typical structures include an air cavity, a dielectric resonator loaded cavity, and a metal resonator loaded cavity. The air cavity is disclosed in a paper by A. E. Atia, et al. entitled "Narrow Bandpass Waveguide Filters", IEEE Trans. on Microwave Theory and Techniques, Vol. MTT-20, pp. 258-265, April 1972. The dielectric resonator loaded cavity is disclosed in a paper by S. J. Fiedziuszko entitled "Dual Mode Dielectric Resonator Loaded Cavity Filters" Trans. on Microwave Theory and Techniques, Vol. MTT-30, pp. 1311-1316, September 1982. The metal resonator loaded cavity is disclosed in", U.S. Pat. No. 5,484,764 entitled "Plural-mode Stacked Resonator Filter Including Superconductive Material Resonators", issued Jan. 16, 1996 to the assignee of the present invention, and in an article by Chi-Wang, et. al. entitled "Dual-Mode Conductor Loaded Cavity Filters", IEEE Transactions on Microwave Theory and Techniques, Vol. 45, pp. 1240-1246, August 1997.
In analyzing available field solutions for the sheath helix, the present inventors have found that these solutions are remarkably similar to the field solutions for a dielectric rod waveguide. Therefore, it has been determined by the present inventors that in many microwave structures utilizing devices that are based on the use of a dielectric waveguide (e.g. dielectric resonators), the use helical resonators offers the possibility of significant miniaturization, especially at lower microwave frequencies.
It would therefore be desirable to have microwave dual mode filters that are smaller in size and lighter weight than comparable conventional filters. It would be desirable to have microwave dual mode filters that have a high Q factor in a miniature package. It would be desirable to have microwave dual mode filters that are voltage or current tunable.
Accordingly, it is an objective of the present invention to provide for improved single mode and dual mode filters. It is another objective of the present invention to provide for improved microwave filters for use at relatively low microwave frequencies. It is another objective of the present invention to provide for improved single mode and dual mode filters that may be used in wireless and personal communication system (PCS) devices It is yet another objective of the present invention to provide for improved single mode and dual mode filters that have a high Q factor and are implemented in a miniature package. It is yet another objective of the present invention to provide for improved single mode and dual mode filters that are voltage or current tunable.